Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current [Abstract]
Federal	$ 17,761	$ 14,537	$ 48,186
State	2,068	1,237	2,366
Deferred [Abstract]
Federal	16,256	18,394	(7,349)
State	2,444	2,769	(1,103)
Income tax provision	38,529	36,937	42,100
Reconciliation of provision for tax from the federal rate to the effective tax rate [Abstract]
Income tax computed at statutory tax rate	56,732	53,899	55,784
Tax exempt interest	(5,612)	(5,222)	(5,150)
Nondeductible interest expense	107	121	144
State income taxes, net	2,933	2,604	821
Income tax credits	(15,212)	(15,755)	(9,255)
Other	(419)	1,290	(244)
Income tax provision	38,529	36,937	42,100
Statutory federal income tax rate (in hundredths)	35.00%	35.00%	35.00%
Deferred tax assets [Abstract]
Loan purchase accounting	38,471	63,048
Other real estate	36,086	47,981
Allowance for loan losses	31,241	29,102
Deferred compensation	20,825	20,000
Realized built in losses	15,907	9,182
Securities	15,443	17,741
Pension and other postretirement benefit plans	14,904	7,511
Nonaccrual loans	3,372	4,452
Stock-based compensation	3,077	3,357
Other	17,833	14,771
Gross deferred tax asset	197,159	217,145
Valuation allowance	(8,650)	(16,000)
Deferred tax asset net of valuation allowance	188,509	201,145
Deferred tax liabilities [Abstract]
Goodwill and other identifiable intangibles	28,010	29,809
Premises and equipment	20,813	21,543
Unrealized gains on securities available for sale	8,627	1,969
Mortgage servicing rights	7,747	8,145
Securities	1,353	1,270
Leases	625	3,290
Other	3,900	4,642
Gross deferred tax liability	71,075	70,668
Net deferred tax asset	117,434	130,477
Changes in unrecognized tax benefits [Roll Forward]
Balance at beginning of period	1,779
Increases due to tax positions taken during the current year	670
Increases due to tax positions taken during a prior year	60
Decreases due to the lapse of applicable statute of limitations during the current year	(306)
Decreases due to settlements with taxing authorities during the current year	(43)
Balance at end of period	2,160	1,779
Accrued interest, net of federal benefit, at end of period	437
Unrecognized tax benefits that would impact the effective tax rate, if recognized, at end of period	$ 1,603